Various approaches to automated or semi-automated three-dimensional ("3D") object production or Rapid Prototyping & Manufacturing (RP&M) have become available in recent years, characterized in that each proceeds by building 3D objects from 3D computer data descriptive of the objects in an additive manner from a plurality of formed and adhered laminae. These laminae are sometimes called object cross-sections, layers of structure, object layers, layers of the object, or simply layers (if the context makes it clear that solidified structure of appropriate shape is being referred to). Each lamina represents a cross-section of the three-dimensional object. Typically each successive lamina is formed and adhered to a stack of previously formed and adhered laminae. In some RP&M technologies, techniques have been proposed which deviate from a strict layer-by-layer build up process wherein only a portion of an initial lamina is formed and prior to the formation of the remaining portion(s) of the initial lamina, at least one subsequent lamina is at least partially formed or, in some techniques, at least a recoating process for forming the at least one subsequent lamina is performed.
According to one such approach, a three-dimensional object is built by applying successive layers of unsolidified, flowable material to a working surface, and then selectively exposing the layers to synergistic stimulation in desired patterns, causing the layers to selectively harden into object laminae which adhere to previously formed object laminae. In this approach, material is typically applied to the working surface both to areas which will not become part of an object lamina, and to areas which will become part of an object lamina. Typical of this approach is Stereolithography (SL), as described in U.S. Pat. No. 4,575,330, to Hull. According to one embodiment of Stereolithography, the synergistic stimulation is radiation from a UV laser, and the material is a photopolymer. Another example of this approach is Selective Laser Sintering (SLS), as described in U.S. Pat. No. 4,863,538, to Deckard, in which the synergistic stimulation is IR radiation from a CO.sub.2 laser and the material is a sinterable powder. As the above two examples use eletromagnetic radiation as the source of stimulation, they may be classified as photo based stereolithography. A third example of stereo lithography is Three-Dimensional Printing (3DP) and Direct Shell Production Casting (DSPC), as described in U.S. Pat. Nos. 5,340,656 and 5,204,055, to Sachs, et al., in which the synergistic stimulation is a chemical binder (e.g. an adhesive), and the material is a powder consisting of particles which bind together upon selective application of the chemical binder.
According to a second such approach, an object is formed by successively cutting object cross-sections having desired shapes and sizes out of sheets of material to form object laminae. Typically in practice, the sheets of paper are stacked and adhered to previously cut sheets prior to their being cut, but cutting prior to stacking and adhesion is possible. Typical of this approach is Laminated Object Manufacturing (LOM), as described in U.S. Pat. No. 4,752,352, to Feygin in which the material is paper, and the means for cutting the sheets into the desired shapes and sizes is a CO.sub.2 laser. U.S. Pat. No. 5,015,312 to Kinzie also addresses building object with LOM techniques.
According to a third such approach, object laminae are formed by selectively depositing an unsolidified, flowable material onto a working surface in desired patterns in areas which will become part of an object lamina. After or during selective deposition, the selectively deposited material is solidified to form a subsequent object lamina which is adhered to the previously formed and stacked object laminae. These steps are then repeated to successively build up the object lamina-by-lamina. This object formation technique may be generically called Selective Deposition Modeling (SDM). The main difference between this approach and the first approach is that the material is deposited only in those areas which will become part of an object lamina or a support lamina. Typical of this approach is Fused Deposition Modeling (FDM), as described in U.S. Pat. Nos. 5,121,329 and 5,340,433, to Crump, in which the material is dispensed in a flowable state into an environment which is at a temperature below the flowable temperature of the material, and which then hardens after being allowed to cool. A second example is the technology described in U.S. Pat. No. 5,260,009, to Penn. A third example is Ballistic Particle Manufacturing (BPM), as described in U.S. Pat. Nos. 4,665,492; 5,134,569; and 5,216,616, to Masters, in which particles are directed to specific locations to form object cross-sections. A fourth example is Thermal Stereolithography (TSL) as described in U.S. Pat. No. 5,141,680, to Almquist et. al. All of the patents referenced above in this section are hereby incorporated by reference as if set forth in full.
In some typical practices of photo-based stereolithography, objects or "parts" are built on a layer-by-layer basis, where each layer represents a thin cross-section of the part to be formed. Early approaches to stereolithographic part building were based on the complete filling of layers (e.g. substantial polymerization of all regions of a cross-section to a thickness at least as deep as the layer thickness). This filling was either done by the selective scanning of a pencil of light using a focused or defocused beam, or by flood exposure of the appropriate cross-sectional image. The pencil of light approach strictly used complete filling of cross-sections based on the scanning of adjacent overlapping vectors or raster lines until the entire cross-sectional pattern was cured.
Other sterolithographic techniques used an internal lattice of partially cured building material ("cross-hatch" or "hatch") in place of completely filling the successive cross-sections. The internal structures primarily consisted of cross-hatch separated by untransformed building material (e.g. liquid photopolymer or the like). In this approach, the outer and inner edges of each layer may be solidified by scanning of what are called "boundary vectors" (also termed, "boundaries" or "border vectors" or "borders") to form boundary lines or zones. These lines or zones separate the interior object regions (i.e., regions designed to be solid) of cross-sections from exterior non-object regions (e.g., regions designed to be hollow). Cross-sections or portions of cross-sections that bound external regions of the part are completely solidified by exposing skin fill data (termed "fill" or "skin").
The skin, crosshatch, and borders may trap untransformed building material (e.g. liquid photopolymer) internally in the part structure and hold it in place while the part is being created. The trapped untransformed building material (e.g. liquid photopolymer) and at least partially transformed building material (e.g. at least partially cured polymer) which forms the boundaries, hatch, and skin may be brought to full transformation (e.g. polymerization) in a later process know as "post curing". For additional information on post-curing, see U.S. patent application Ser. No. 07/415,134 now abandoned.
Fairly extensive post-curing can be required when the internal cross-hatch lattice only defines discrete x-z and y-z, planes, or the like, which are separated from each other by more than the width cured by a beam, as in such cases long vertical corridors of unpolymerized material remain substantially uncured until post-processing.
In some instances, down-facing features are given both hatch and skin fill, thereby increasing the possibility of overexposure of the regions where the hatch and fill coincide.
One build style that utilizes hatch as opposed to solid fill is called QUICKCAST.TM.. An in-depth explanation of the QUICKCAST.TM. build style and its uses is found in U.S. Pat. No.5,256,340 to Allison; U.S. patent application, Ser. No. 08/233,026, filed Apr. 25, 1994, now abandoned; and U.S. patent application, Ser. No. 08/428,950, filed Apr. 25, 1995, now abandoned. This patent and these two applications are assigned to the assignee of the present invention. These references are incorporated by reference as though fully set forth herein.
In general, the QUICKCAST.TM. build style hollows a solid object and builds the interior of the object, for support purposes, with an internal support structure (e.g., hatch structure). An early QUICKCAST.TM. build style used an equilateral triangle hatch structure. In this embodiment, the hatch structure was offset between periodic layers. Typical build parameters used resulted in a void ratio of approximately 80%. In this context, void ratio is defined as the difference between 100% and the percentage of the ratio of the weight of the QUICKCAST.TM. object versus the weight of the associated object built as a solid.
A typical use for objects built with the QUICKCAST.TM. style is as patterns for investment casting. When used as patterns for shell type investment casting, the part must be burned out of a ceramic mold without cracking the mold. Though the above described triangular build style resulted in a reasonable drainage of liquid from the object, it was common for some liquid to remain trapped inside the object. The excess strength of the object and the inability to drain all of the liquid out resulted in an excess number of molds cracking prior to object burn out and associated inability to use the objects for their intended purpose.
Another QUICKCAST.TM. build style used a square hatch pattern and typically resulted in a void ratio in the mid to low 80s. Not only was the void ratio somewhat higher, as a square pattern is weaker than a triangle, it is believed the structure collapses more easily. This structure resulted in an improved success rate for creating ceramic shells though an excessive number of failures was still common. The success rates were still below the common 95% rate for wax investment casting patterns.
One attempt to improve the drainage of the liquid from the object is the utilization of a broken hatch style to form a discontinuous hatch structure, wherein the structure, a hexagon, is formed using opposite sides of the hexagonal pattern on successive groups of layers. The patterns for successive groups of layers are depicted in FIGS. 1a, 1c and 1e.
Although this broken hatch style increased the drainage of liquid from the object, the building of the hatch style was not reliable as each element of structure on the first layer above a transition from one pattern to another is only supported by its endpoints.
In light of the foregoing, a need exists for a more reliable build style that increases the drainage of liquid from the object.